An organic electroluminescent element (hereinafter, also referred to as an organic EL element) is a light emitting element which has a constitution in which a light emitting layer containing a light emitting compound is sandwiched between a negative electrode and a positive electrode, in which excitons are generated in the light emitting layer by recombination of holes injected from a positive electrode and the electrons injected from a negative electrode according to application of electric field and light (fluorescence or phosphorescence) is emitted when the excitons are inactivated. The organic EL element encompasses any individual element composed of an organic material film having a thickness of mere about submicrons at most between the electrodes and can emit light at a voltage of about several volts to several tens of volts, and thus its application as a next-generation flat display or a lighting device is anticipated.
Princeton University has developed an organic EL element for practical application and has reported an organic EL element using phosphorescent light emission from an excited triplet state (see, for example, Non Patent Literature 1). Materials emitting phosphorescence at room temperature have been extensively studied since then (see, for example, Patent Literature 1 and Non Patent Literature 2).
Furthermore, the viable emission efficiency in organic EL elements using phosphorescent light emission recently discovered is about four times larger in principle than those of conventional elements using fluorescent emission. Researches and developments of layer constitutions and electrodes of light emitting elements, as well as the developments of materials for the elements, have been conducted all over the world. For example, synthesis of many compounds, mainly, heavy metal complexes such as iridium complexes, has been investigated (see, for example, Non Patent Literature 3).
Although organic EL devices using phosphorescent light emission have significantly high potential as described above, they have quite different technical issues from organic EL devices using fluorescent emission, that is, the control of the position of emission center, in particular, how recombination can be performed within a light emitting layer and how stable emission of light by recombination in the light emitting layer can be achieved, which is a key technical challenge for determining the efficiency and lifetime of an element.
Under such a circumstance, a multilayered laminate element having a light emitting layer, a hole transport layer adjoining the light emitting layer (provided on the positive electrode side of the light emitting layer), and an electron transport layer adjoining the light emitting layer (provided on the negative electrode side of the light emitting layer) have been well-known (see, for example, Patent Literature 2). Many mixed layers containing a host compound and a phosphorescent light emitting compound as dopants are used for a light emitting layer.
Meanwhile, in terms of the materials, a material which has a high carrier transporting property or is thermally or electrically stable is required. In particular, for using blue phosphorescent luminescence, development of side materials that can be applied and fine control of light emitting center are strongly required because a blue phosphorescent light emitting compound itself has high triplet excitation energy (T1).
As a representative blue phosphorescent light emitting compound, FIrpic is known, and short wavelength has been achieved by fluorine substitution of phenylpyridine of a main ligand and use of picolic acid as a sub-ligand. Those dopants are combined with carbazole derivatives or triarylsilanes as a host compound to constitute an element with high efficiency. However, because the light emitting lifetime of the element is significantly deteriorated, there has been a demand for an improvement of such trade-off.
Recently, as a blue phosphorescent light emitting compound having high potential, a metal complex with specific ligand is disclosed in Patent Literatures 3 and 4. However, although the luminance efficiency and light emitting lifetime have been improved, the organic EL element described in those literatures has a problem in terms of thermal stability of a metal complex as a material of an organic EL element. As such, due to an occurrence of decomposed products during forming an organic layer by deposition using such metal complex, there is a case in which the lifetime of the element is compromised.
Further, from the viewpoint of using it for a display or a lighting device, the organic EL element is also required to have long term stability. With regard to a light emitting layer of an organic EL element, a state within the light emitting layer changes when light emitting is allowed to occur continuously over a long period of time or under an environment of high temperature.high humidity due to concentration quenching as caused by crystallization or aggregation of light emitting dopants, quenching as caused by interaction between excitons, or the like. As a result, a problem may arise in that the driving voltage increases or deterioration of element performance like decreased luminance brightness occurs. In this regard, there is no description at all in Patent Literatures 3 and 4 regarding the long term stability. Thus, it was found that a further improvement is in need.